A necessary step in creating digitized image signals from analog sources is the quantizing, or sampling of the dynamic range, of these image signals into discrete levels. In addition, spatial (or temporal) sampling is also performed. Given limited resources for storing, transmitting, reproducing, processing, or otherwise manipulating a digitized image signal, it is desirable to reduce the spatial resolution and/or the number of quantization levels (dynamic range resolution). Reducing the spatial resolution reduces the frequency response of the digital image signal, while reducing the number of quantization levels results in contouring and other reproduction artifacts.
It is well know from an article entitled "PCM Encoded NTSC Color Television Subjective Tests" by A. A. Golberg, JSMPTE August, 1973, p.p. 649-654 that a square wave or random signal can be added to a signal before quantizing to reduce the contouring that can result from this quantizing and then a subsequent low pass filtering of this combined signal can be performed to reduce the visibility of the quantization noise. This technique has been described in various prior art publications, for example, see U.S. Pat. No. 4,825,285 entitled "HYBRID ENCODER", by Speidel et al. wherein it is noted that the low pass filtering operation yields a lack of picture definition which is however, less disturbing than the above mentioned disturbances caused by quantization errors.
A patent of particular interest for building on the aforementioned article is U.S. Pat. No. 4,334,237 entitled "ADAPTIVE AMPLITUDE AVERAGING FOR WEIGHTING QUANTIZING NOISE" by Reimeier et al. wherein a method and an apparatus are disclosed for determining if only low frequency information is present. This method and apparatus are used to determine when this low pass filtering operation should be performed. In the detailed description of this method and apparatus it is noted that in the case where the averaging or integration is performed (i.e. the low pass filtering operation), that the maximum error is one half of the quantizing step. It is further noted that in areas of high frequency information that the maximum error is increased to one and one-half of a quantizing step since the signal, which by virtue of the disclosed method has a one half quantization level magnitude square wave added to it, is not averaged.